Subtractive colours start with light, presumably white light. Coloured inks, paints absorb some wavelengths of light and reflecting or transmitting others. The three primary subtractive colours are yellow, magenta and cyan.
The yellow pigment absorbs blue light allowing red and green to pass through reflecting from its surface. We perceive the red and green light and see yellow.
The magenta pigment absorbs green light allowing red and blue to pass through or reflect from its surface. We perceive the red and blue light and see magenta.
The cyan pigment absorbs red light allowing green and blue light to pass through or reflect from its surface. We perceive the green and blue and see cyan.
Yellow ink absorb blue light, cyan ink absorbs red light. The only light that remains is green. Green is the subtractive secondary colour achieved by mixing two primary.
Yellow ink absorbs blue light, magenta ink absorbs green light. The only light that remains is red. Red is the subtractive secondary colour achieve by mixing two primary.
Magenta ink absorbs green light, cyan ink absorbs red light. The only light that remains is blue. Blue is the subtractive secondary colour achieved by mixing two primary.
When all three light are in the mix all three lights are absorbed (blue, green, red) therefore we cannot see any light in effect see black.
Yellow + Magenta + Cyan = Black
The printing industry use CMYK colour space, K representing not quite black = when all 3 subtractive colours are combined.